The present invention relates to a device for the selection of hooks in a dobby of a loom.
Such a device is used in the textile industry for the purpose of allowing production of fashioned woven fabrics, that is to say requiring a very large number of warp yarns which have to be raised or lowered. Such woven fabrics constitute, for example, paintings, tapestries or other pictures for which hand looms are still often used.
It has been proposed in document FR 2,476,694 to produce a warp-yarn control device for the purpose of opening the shed, by dividing this device into separable modules each including heald shafts which pass through a first board provided with bars and a second board provided with holes. The heald shafts each carry a shoe and a hook. When electromagnets attract heald shafts, the hooks of those which are called are moved by the first board while those which are not called follow the translational movement of the second board. Thus, the shed is opened by the translational movement of the two boards.
Such a dobby is very complicated and requires great accuracy in producing all its elements for the purpose of guaranteeing the relative position of each heald shaft with respect to its electromagnet and its lifting board. This dobby is provided with a large number of movable elements in such a way that, in order to prevent any premature wear and for the purpose of obtaining the accuracy essential for producing all the elements, such a dobby ends up being very expensive.
Document EP-0,108,700 discloses a device for the selection of hooks in a dobby which includes a bottom board carrying the hooks and an upper part which supports the selection device and can move with respect to the bottom board, this device including pieces provided with a catch, each of these pieces corresponding to one hook of the bottom board and the position of the catch with respect to the hook being controlled by an electromagnetic device having at least one coil, in such a way that, when the bottom board moves away from the upper part, the catch, depending on its position, entrains or does not entrain the hook with it.
In this device, the pieces provided with a catch are elastic heald shafts. The latter are elastically deformed when the bottom board moves towards the upper part. The heald shafts then come to bear against a pole piece of a permanent magnetic circuit. The heald shafts are held in this position, which does not allow retention of the hooks of the bottom board. Corresponding to each heald shaft there is therefore a coil which is excited when the heald shaft has to retain a hook of the bottom board, allowing the heald shaft to resume its non-deformed position.
When magnetic dust particles become stuck on a pole piece of the permanent magnetic circuit, it may happen that the corresponding heald shaft is then no longer held in its deformed position and therefore this heald shaft retains each time the corresponding hook of the bottom board. This then causes a defect in the weaving of the woven fabric.
Furthermore, in order to deform the elastic heald shaft, an inclined ramp is provided in the region of each hook of the bottom board. In order to avoid a shock when the bottom board moves towards the upper part, the slope of this ramp is small. In order to obtain sufficient deformation of the heald shaft, the relative stroke between the bottom board and the upper part must be relatively long. Since the stroke is long, the time to cover it is also long and, consequently, the time for inserting the weft is limited.